I had the same problem as "user5412" viz the zoom motor hanging up, which disables the flash until the flash is power cycled. Lost some "money shots" that way. Got a replacement that hasn't done it yet.

I disagree with "TuanHA", you can do high speed sync, but the flash has to be put in that mode. See the manual.

The biggest issue is that multi-flash setups in ETTL on Canon cameras don't work. See: